Posted
by
samzenpus
on Wednesday March 16, 2011 @02:00PM
from the space-sharks dept.

Hugh Pickens writes "MIT Technology Review reports that various ideas have been floated for removing space junk, most of them hugely expensive, but now James Mason at NASA Ames Research Center has come up with the much cheaper option of zapping individual pieces of junk with a ground-based laser, to slow them down so that they eventually de-orbit. Mason estimates that a device to test the reversal of the Kessler syndrome could be put together for a million dollars, which would have to be shared by many space-faring nations, to avoid the inevitable legal issues that using such a device would raise. 'The scheme requires launching nothing into space — except photons (PDF) — and requires no on-orbit interaction — except photon pressure. It is thus less likely to create additional debris risk in comparison to most debris removal schemes,' writes Mason. 'Eventually the concept may lead to an operational international system for shielding satellites and large debris objects from a majority of collisions as well as providing high accuracy debris tracking data and propellant-less station keeping for smallsats.'"

Ability to blind populations on the ground in peacetime. This laser would have to be powerful. Satellites are irregularly-shaped and have flat reflective surfaces. See where I'm going with this?

Are you suggesting that governments will blind, say, protesters or peaceful people they don't like who happen to be staring at the lasers or at sattelites, which will reflect the beams into their eyes? Wow, that WOULD be a nightmare scenario, those evil bastards! So much wasted tax money! They should really stick to the low-tech grabbing them, putting them in a van, and burying them in an unmarked grave. It's SO much more efficient.

In fact, what I'm really saying is, there's really no safe way to use this thing on a random piece of space junk. Far too great a chance of half of its energy being reflected back into the face of one of the millions of people oooing and ahhhhing on the ground when it's fired into the sky.

That's not how that works. The likelihood of that happening is nihil. It takes a huge amount of energy to get a laser to paint the target, let alone actually cause a perceptible change in velocity, the reason being that light tends to get scattered as it passes through the atmosphere. So, in order to bounce a laser off a bit of debris in space and get it in the eyes of anybody, is more or less impossible, when you account for the loss of energy, the tiny target on both ends.

Well there is also the fact that those laser pointers paint a target half the size of the airplane at a mile out. Even were they to aim correctly, the beam would be too diffuse to be more than a nuisance.

How is this not +5 Insightful already? Does anyone really think we can split or stop metal parts by shining light on them? It was a slightly less retarded idea when all they wanted to do was burn a tiny little hole into aircraft and nukes, but this is ridiculous.

The theory of it works, kinda-sorta, but only for objects for which you know all of the characteristics, and environments where everyone and everything that's vulnerable to backscatter is protected. Space-junk and satellites are basically randomized, especially when they're in a state where you have to use a ground-based laser to try to nudge them into more-quickly-deteriorating orbits.

They might as well flash this thing at a disco ball and tell everyone to get a good look.

What? You insist on doing arithmetic? Something like P = I/c (within a factor of 2) for the radiation pressure exerted by light? So that if we make a very generous assumption of a kilowatt in a single square centimeter, we have 10^3/3x10^8 \approx 3 x 10^-6 Pa. We multiply by 10^-4 square meters (one square centimeter) and get 3 x 10^-10 Newtons. Take into account attenuation in the atmosphere and the fact that the beam is wiggling all over the place because of atmospheric thermal ripple and lensing, a

If you read the article, they themselves say this would never be anywhere near enough power to de-orbit anything. The purpose of this is merely to nudge things into another orbit, specifically one that does not cross that of the satellite they are trying to protect. Now whether those minute changes in velocity a week out would be enough to significantly change the trajectory, and whether they would even be able to predict an orbit that accurately that far out, I can't say. It's been too long since my orb

Actually yes, repeatedly. I've even published papers on nonlinear optics, although that is as irrelevant as your comment. What specific parts of the results are supposed to surprise me?

Wait, I'll go upstairs and get a laser. There. Yup, fortunately it still works (my sons tend to like to play with them and run down the batteries). Lens, lens, I need a lens. Wait! I know! My reading glasses are an actual lens! Experiment: Shine laser on wall no glasses, a sharp point with the usual spackle. Shin

My understanding is that this is an old idea [bbc.co.uk], and the core method is definitely ablation [arxiv.org] not the negligible force of radiation pressure.

This system is intended to stop particles that are between 1 and 10 cm in diameter. Currently deployed technology allows for reliable ground tracking of debris that is approximately (supposedly) 10 cm, though proposed laser based tracking systems would detect debris in the 1 cm range. Presuming a 5x5x5 cube of solid steel, that's more like 1kg. Individual hobbyists are c

Only the idea as described in the article. The questions I'd have for the very different method you describe is how you plan to find a ~1 kg chunk of mass between 1 and 10cm in size and hit it with a laser. That's something like 0.002-0.02 arcseconds, right? Pretty damn good shooting required (NASA or not) and you also have to deal with the spreading of the beam at an oblique traversal of the atmosphere, although I have no immediate sense of the order of magnitude of this. However, 0.02 arcseconds of sp

Does anyone really think we can split or stop metal parts by shining light on them?

I had a guaranteed military sale with ED 209. Renovation program. Spare parts for 25 years. Who cares if it worked or not? -- Dick Jones, Robocop

Are you kidding? This idea is brilliant. It's an impossible goal that demands huge amounts of funding which the military will find as appealing as crack-soaked catnip served on hookers. It's not a blank check -- it's a limitless credit card that will never get declined. They've been buying this schtick since Reagan. When this idea plays out -- finally, a few decades from now -- we'll just move on to promising them phasers, then blasters and light sabers.

The idea is not to blast debris out of the sky. The idea is to change the orbit ever so slightly using photon pressure. The laser is fired as the object comes over the horizon until the object is at the zenith. This has two effects. The obvious, since the laser is only fired until the object is at zenith it pushes against the object's orbital motion, effectively lowering it's orbital velocity. Less obvious, it pushes the satellite 'up' away from the earth. Since you aren't actually increasing orbital

Forty years ago a Hughes engineer showed me a small block of steel that had been cut in half rather irregularly, as if an apprentice welder shined an oxy acetylene cutting torch on it. I enquired; he said "It's really not all that bad a cut, considering it was done from ten miles away through fog."

How is this not +5 Insightful already? Does anyone really think we can split or stop metal parts by shining light on them? It was a slightly less retarded idea when all they wanted to do was burn a tiny little hole into aircraft and nukes, but this is ridiculous.

The laser would have to be focussed pretty accurately onto the satellite to be effective. After passing through a few hundred kilometres of atmosphere twice, and being hugely out of focus (i.e. diffused over a large area), the most you'd see from the ground would be a bright flash, no worse than an Iridium flare.

...except for the part where changing air currents and densities (the same reason why starlight seems to twinkle at night) distorts lasers, so there are "tracer" low-powered lasers so that the system can properly shape, focus, and aim the main beam. You think that reflected laser is going to magically focus itself after bouncing off of some junk to blind someone? Come on folks, this is basic SDI/ABL stuff.

There are already websites that track what are called "Iridium Flares" where the sun reflects off one of the boxy, shiny AT&T Iridium satellites. Focus the beam a little more and you could accomplish some fairly serious eye damage. However, aiming such a thing at a specific target for any length of time would be damned near impossible.

If you wanted to truly permanently blind a populace, issue each of your people a 1kw aiming laser for their rifles. If you want to temporarily blind them, set up a coup

Actually, less than a mW of sustained exposure would be sufficient to cause permanent damage. Somewhere up to around 5mW is considered "safe" as your blink reflex will happen fast enough to protect your eyes. Also, we've already got this nice stellar object about a hundred million miles out that will cause damage if you stare at it.

At just a few kW, there is absolutely no risk of harm from this. The beam will scatter and spread on the way up through the atmosphere, hitting the object at something far les

Looking at this from a different angle, the maximum intensity of a standard class IIIa laser pointer is 2.5mW/cm. In order to achieve that density at that power rating, the reflected beam must be no larger than 50ft wide. That's two hundredths of an arcsecond. We just aren't that good, and through the atmosphere it's simply not possible.

I actually hadn't thought of that bug good point. What did occur to me was the exact opposite of that coin. Meaning, couldn't this then be used as a was to change orbits or to provide a boost in altitude?

To avoid legal incidents we will be mounting the lasers in international waters. We will be subsidizing costs by using existing biological life-forms, mainly sharks, as the key base for the laser installation. Aiming the devices will also utilize the shark's keen sense of smell to identify and destroy decaying orbital installations.

Meh. This idea has been bouncing around for a while now:
http://www.youtube.com/watch?v=UN8YIR60Ij0 [youtube.com]
Though the application demonstrated in the video was for a slightly different purpose, it would be an additional benefit, should the need arise.

Pay big bucks to bag an elephant? Or a rare Antarctica albino bearded clam tiger? Offer folks who have unreasonable amounts of money the opportunity to bag a satellite. It looks great mounted, up on the wall of the Africa room, next to Bambi's head. The profits could go to getting NASA back into action.

We'll aim the laser on the satellite, Sir . . . all you have to do is pull the trigger . . .

... how could photons originating from the earth pushing on an orbital object do anything other than move it away from the earth?

Easy:

1) Hit it more on the leading than the trailing side. The ejected material leaves primarily forward, producing a deceleration. (This even works with just photon pressure bouncing from an ideally reflective object.) Yes you always CAN hit the leading side: The laser is not at the center of the Earth so just shoot when it's coming somewhat toward you.

Any law that forbids cleaning up space? As there is no international convention about at which height airspace ends and space starts, all should be fine if the country using such a laser only shoots stuff above it's own territory.

I commented elsewhere on the actual numbers, which show quite clearly that even a 5 kw laser would exert at most completely irrelevant forces on any object large enough to actually see from earth and hence target -- accelerations on a good day of 10s of microns per second per year of radiation pressure. Having RTFA and noted all of the corrections by the authors (of the idea, if I understand things correctly) it is still an enormously stupid idea. What part of piconewton scale forces is difficult to understand?

I give this one as an assignment for my intro physics classes -- suppose you have a megawatt laser with a beam 1 cm^2 across and mount it on the rear of your spaceship to use as a drive. Wow, a whole million watts of power! Surely that will provide the ship with all kinds of thrust!

Sure, if all kinds of thrust is a few micronewtons.

You'd get more thrust -- and probably more net delta-vee for any acceleration time you are willing to wait -- if you simply took the laser to the door of your capsule and threw it, as hard as you can, away.

Yeah, too bad they're relying entirely on light pressure, and aren't planning to take advantage of, say, the possibly-catastrophic outgassing they might get from heating one side of an object. I mean, obviously one of their main concerns is going to be not damaging the objects they're trying to destroy.

In the after-article article they make it clear that they're not trying to destroy the objects (which they cannot do in any event), and that they recognize that it would take decades to actually knock something out of orbit (a vast underestimate -- try centuries). They allege -- and I'm still having a hard time seeing it but it is barely possible -- that they can divert the orbit of something enough to prevent a collision. The after article also makes it clear that the actual cost of building a laser syst

In the after-article article they make it clear that they're not trying to destroy the objects (which they cannot do in any event)

Unless they're very large objects (very large indeed!) I would think that trying to deorbit them would tantamount to destroying them. But no, I understand that the lasers aren't intended to directly destroy the objects, which is why I mentioned outgassing.

They also clearly state that they are relying on light pressure, not outgassing or heating in general.

Ah, well in that case I retract my comment, but as you say, the basic idea of moving the objects enough to avoid collisions is within the realm of the possible, and I would tend to suspect that they've had people with at least a rudimentary knowledge of p

The light from the sun seems irrelevant, as it's not purposeful, though I imagine they would have to take it into account when calculating when and how (and whether) to aim their lasers.

Interestingly (I did some more looking at this and found some articles via Google about drag forces) the state of the sun is very important to the drag force that eventually does deorbit the satellites. My point was that sunlight at low earth orbit has an intensity of roughly 1.4 KW/m^2, which is comparable to the laser

Well, FWIW, the Sun does affect satellite orbits. Enough that they usually (used to?) have hydrazine rockets to adjust station keeping.

OTOH, it doesn't affect them quickly.

OTTH, if you de-circularize the orbit very much (and it's in LEO), then atmospheric drag will de-orbit it.

If it's not in LEO, then you probably can't do any better than just move it into an orbit where it won't hit anything interesting. Preferable into one where it will collide with another piece of junk at 1 mm/sec or less. Then, if y

But you can use such arrays against paint, rust, frozen water, and the other dust cluttering LEO. And that should be quite detectable as "glitter" in the ordinary spy satellite monitoring of LEO, which is taken from LEO and has less relative velocity to cope with and less atmospheric distortion. So I think your premise of being able to "see it from Earth" has nothing to do with most of this debris.

a) The article methodoology explicitly claims to be using light pressure only.

b) If they can actually hold a 5 KW laser on target at a single point on the object it might heat it up to where ablation occurs, and yes that would produce easy orders of magnitude greater (if still tiny) thrust. However, I don't think they can come close to achieving this and they'd have to work quite hard to convince me before I was willing to drop $50-100 million dollars on the experiment. Atmospheric ripple and other so

Are sharks really accurate enough to shoot down orbital debris with a head mounted laser? If they released some piranha into the sharks tank, would that be sufficient to destroy Tokyo (is a thrashing shark with a laser as powerful as Godzilla)?

I'd rather use something without recoil for that, unless we want to add some astronauts to the debris.

Oh, and I faintly remember a SciFi story about two enemies on Mars, where one misses shooting his last bullet at the other. The other one just kept talking with him, until the bullet, being fast enough to make a full orbit, hit the guy who shot it in his back. Or at least something like that, must be about 25 years ago. Anyone who knows that story here?

First, it doesn't work like that. Even without an atmosphere gravity would still make the bullet fall to the ground. To make it actually go in orbit like that, the bullet would have to reach escape velocity, which is 5 km/s on Mars. That's the heck of a gun.

Second, Mars does have an atmosphere, which would introduce extra difficulties.

Third, assuming they're on the ground, Mars isn't entirely flat and it'd almost certainly hit elevated terrain before it could make a w

The general idea should still apply though. Some googling suggests the fastest bullets are about 1/3 of that speed. It's probably hard to get much faster than that, as wherever you are, the third Newton's Law still applies and it's rather crucial not to break your own arm when shooting the gun.

To do that you need: - A bullet velocity between escape velocity and circular orbit at that radius.
- No atmosphere (which would make the orbit decay enough that it wouldn't hit the shooter).
- A planet with no appreciable mass concentrations to perturb the orbit.
- A shot fired nearly dead horizontal. (Too low and it hits ground before it orbits. Too high and the orbit goes through the ground behind the shooter.)